Question

It should match all the following examples.

aBCd22
a2b2CD
22aBcD

Answer 1

A compact version, using the positive lookahead operator:

^(?=(.*[0-9]){2})(?=(.*[A-Za-z]){4})[A-Za-z0-9]{6}$

Explanation:

(?=(.*[0-9]){2}) # asserts that you have 2 digits
(?=(.*[A-Za-z]){4}) # asserts that you have 4 characters
[A-Za-z0-9]{6} # does the actual matching of exactly 6 alphanumeric characters

A simple test case, in Python:

import re
rex = re.compile('(?=(.*[0-9]){2})(?=(.*[A-Za-z]){4})[A-Za-z0-9]{6}')
tests = ['aBCd22', 'a2b2CD', '22aBcD', 'asd2as', '', '201ABC', 'abcA213']
for test in tests:
print "'%s' %s" % 
       (test, "matched" if rex.match(test) != None else "didn't match")

Output:

'aBCd22' matched
'a2b2CD' matched
'22aBcD' matched
'asdas' didn't match
'' didn't match
'201ABC' didn't match
'abcA213' didn't match